Balance pistons are often used in turbomachines to manage or control axial thrust loads generally created by pressure differentials along the axial length of the turbomachine shaft. In centrifugal compressors, for example, the balance piston typically includes a disk mounted to the shaft on the outboard side of an impeller, often the final stage impeller. A reference line fluidly connects the outboard side (i.e., the side facing away from the impeller) of the balance piston with process gas provided at a reduced pressure, generally suction pressure. Accordingly, the axial forces directed from the high-pressure impeller outlet toward the low-pressure suction inlet are at least partially offset by the pressure differential being experienced in the opposite direction across the balance piston. Remaining axial thrust loads are typically taken up by one or more axial bearings, which are known and available in a variety of designs.
A challenge inherent to the balance piston solution is that it generally adds an interface between a rotating component and a stationary component. Generally, such interface is sealed using any one of a variety of different types of seals. However, the efficacy of the seal is generally a function of the sealing surface area, and the sealing surface area is limited by the axial length of the balance piston. Moreover, it is generally desirable to limit the axial length of the balance piston, and thus minimize overall shaft length and weight.
Further, gas balance seals are used to prevent contamination or fouling of sensitive seals, such as dry gas seals, with dirty process gas, while allowing sensitive seals on both ends of the shaft to operate at the same pressure. Generally, such gas balance seals are provided by a pair of seals, e.g., labyrinth seals, disposed between the dry gas seals and the balance piston. Clean seal gas is then injected between the labyrinth seals, such that the seal gas leaks across the seals. For one of the labyrinth seals, clean gas flows therepast, with an attendant drop in pressure, toward the balance piston, ensuring that no dirty gas migrates in the opposite direction, toward the dry gas seals. The other labyrinth seal acts as a blow-down seal and provides a required pressure drop, such that the dry gas seals at the high-pressure end of the machine operates at the same pressure as the dry gas seal at the low pressure end of the machine.
While balance piston seals and gas balance seals are generally suitable for a variety of applications, it is commonly desirable to reduce shaft length, thereby increasing stiffness. However, when applied to seals, such reductions in shaft length are generally limited by a trade-off with sealing ability. What is needed is a seal assembly that maximizes sealing surface length while reducing, or at least not substantially increasing, the axial length of the shaft required for the balance piston and/or gas balance seal.